Monica and Itzjbean have given you great advice/opinions....
The fact that a bird doesn't ever lay an infertile egg is not a way to sex them; female birds can go a lifetime and never lay an egg, and if they are by themselves as your bird was, then it's not uncommon at all for them to not lay any infertile eggs.
Also, the behavior between your two birds that you're describing is not indicative of a male/female pair at all, but rather just a "pair" of birds that is very closely-bonded with each other, they could be two males, two females, or a male/female pair, you have no idea with Love Birds unless you have them DNA tested, or if one of them does lay an egg. And as Itzjbean mentioned, the reason your older bird is no longer interested in people is because you got it another bird, which they naturally would rather bond with and be close to than being bonded with people any longer; that's unfortunately what happens whenever people buy a second bird for their first bird, then they lose their bond with their first bird (not always, but a great deal of the time)...So that behavior of lunging, hissing, growling, biting, acting odd outside of the cage now, etc. is all perfectly normal for your older bird now, as they don't want to be near you, but rather with their new friend/mate that is also a bird.
****All of this being said, you have no idea what sex/gender either of your birds are, and it's very likely that your older bird is actually a female...So naturally what comes to my mind as the possible issue is that your birds may be a male/female pair, but in the opposite gender-assignments that you were thinking...So if this bird with the issues you're describing is a female, she could very well be suffering issue with egg-laying/development.
You need to get a Cuttlebone and/or a Mineral Block made for birds and make sure they are both inside the cage at all times, along with a separate bowl of egg-food, as the number one reason that female birds become egg-bound is a low blood Calcium level. this also ends-up with the Calcium being leeched from their bones by their bodies. So that's why the Cuttlebone, the bird Mineral Block, and the separate dish of Egg-Food is so important, because if this bird is a female and she is having egg issues, if she becomes "Egg-Bound", it is 100% fatal without immediate medical intervention by an Avian Vet. Since you don't have access to an Avian Vet unless you drive 2-3 hours away, you need to ensure that both of your birds are getting ample Calcium every single day (which by the way is well worth the drive for the health of your bird, we in the United States often drive hours and hours one-way to get to the nearest Avian Specialist Vet)...
Other than an egg issue, then it does very much sound like a Neurological issue, either locally in the leg, or at the spinal column. The symptoms you describe and the sudden onset of the symptoms do sound very possibly Neurological, like the bird had some kind of accident, or got into a fight with the other bird, and has impinged a nerve, or actually suffered an injury to the back...It happens when you're not looking, and even the closest of bird-pairs sometime fight...Ultimately if it doesn't resolve itself very, very soon at this point, then you need to make the 2-3 hour drive to the Avian Vet, as the bird could be in pain, and the issue may ultimately either continue to get worse, or it may kill the bird eventually...